Minimum requirement is Win2K8-R2 and you will need ADFS. If you have Win2K12 then I would use Win2K12 as there are many new features and improvements made. All you need is a DC with ADFS and Office365 will sync your DC for auth.

The newest release of 2012 offers many small and important improvements.

Standard edition comes with 2 licences for virtual machines. So, if you planning to have less than 8 virtual servers per physical server, it’s the optimal license configuration: a host with Windows Server 2012 R2 Hyper-V and 2 VMs per Standard license.

Are you going to an event? Are you going to be exhibiting at a tradeshow? Talking at a conference? Using a promotional banner in your email signature ensures that your organization’s most important contacts stay in the know and can potentially spread the word about the event.

I would say, No need to have dirsync or adfs for 12 users.
If you have Exchange server on prem you can migrate mailbox using
- Cutover Migration
- Staged Migration

If your email is hosted with third party hosting services. You can migrate mailbox using
- Imap (Only email items will be migrated to Office 365)
calendar and contacts needs to be imported to PST and Exported to individual email account.
- PST migration. Take pst backup of all user and upload the pst file to office 365 user account individually. For 12 user it will take hardly 6 hours.

Any difference / security improvement issues between using AD or dirsync
Any more complicated likely firewall requirements using one compared to the other?

What I want is
0. email everywhere
1. local shares with some but not all staff having access to these shares - to run eg accounts program for a single location. No remote access
2. cloud shared data for all employees with 99% access from staff based in 1 office
3. same data shared when staff are off site - slower access acceptable
4. occasional sharepoint data for staff access from any pc in the world
5. sharepoint data for limited customer access when offered

It all to run as fast as possible ona 4MB crappy line

Flexibility is better than a strict desire to not use what in theory we don't actually need

Does AD make any of this easier for staff control/ease other than point 1.

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